Graptolite-derived organic matter in the Wufeng–Longmaxi Formations (Upper Ordovician–Lower Silurian) of southeastern Chongqing, China: Implications for gas shale evaluation

Abstract

The organic-rich shales of the Wufeng–Longmaxi Formations (Ordovician-Silurian) are one of the most important hydrocarbon source rocks and shale gas exploration targets in China. Graptolites are abundant in these shales, but their contribution to the dispersed organic matter (OM) has been overlooked. This lack of attention could seriously affect the evaluation of OM type, OM maturity, hydrocarbon generation potential, reservoir characteristics and gas content of these shales. Optical microscope and focused ion beam-scanning electron microscope (FIB-SEM) techniques were employed to describe the optical characteristics and micro-nanopores of the graptolites, to determine the contribution of the graptolites and to evaluate the thermal maturity of the Wufeng–Longmaxi Formations.Non-granular graptolites, which show a stronger anisotropy and higher reflectance under polarized light than the granular graptolites, were primarily observed in these sediments. Graptolites account for 20–93% of the dispersed OM in the Wufeng–Longmaxi Formations, and the level of their contribution has been attributed to the radiation of graptolites at the late Ordovician (Kaptian) and the recovery of graptolites at the early Silurian (Rhuddanian). The bireflectance values of the non-granular graptolites fall between 2.22 and 4.32%, indicating a behavior similar to biaxial anthracitic vitrinite. The equivalent vitrinite reflectance (EqVRo) values of the sediments in the Wufeng–Longmaxi Formations, calculated from the mean maximum reflectance of non-granular graptolite, fall in the range of 3.08–4.29%, and these values have a precision that is greater than in previous studies on the Wufeng–Longmaxi Formations. Abundant organic pores were observed in the non-granular graptolites, and they are aligned parallel to the fusellar layers of the graptolites. This indicates that the fusellar layers of non-granular graptolites controlled the development and distribution of organic pores. Such abundant organic pores played a significant role in the accumulation of shale gas in the Wufeng–Longmaxi Formations of the Upper Yangtze Platform.